Short-term variability and mass loss in Be stars

Abstract

Context. Be stars are important reference laboratories for the investigation of viscous Keplerian discs. In some cases, the disc feeder mechanism involves a combination of non-radial pulsation (NRP) modes. Aims. We seek to understand whether high-cadence photometry can shed further light on the role of NRP modes in facilitating rotation-supported mass loss. Methods. The BRITE-Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014–2016. We added observations with the Solar Mass Ejection Imager (SMEI) in 2003–2010 and 118 Hα line profiles, half of which were from 2016. Results. For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed g modes near 1.5 c/d, one slightly lower exophotospheric (Štefl) frequency, and at 0.05 c/d the difference (Δ) frequency between the two g modes. This top-level framework is indistinguishable from η Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The circumstellar (Štefl) frequency alone does not seem to be affected by the Δ frequency. The amplitude of the Δ frequency undergoes large variations; around maximum the amount of near-circumstellar matter is increased and the amplitude of the Štefl frequency grows by a factor of a few. During such brightenings dozens of transient spikes appear in the frequency spectrum; these spikes are concentrated into three groups. Only 11 frequencies were common to all years of BRITE observations. Conclusions. Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the slow Δ variability and/or the atmospheric response to it appears significantly non-linear. As in η Cen, the Δ variability seems to be mainly responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of Δ frequencies may reach the longest known timescales of the Be phenomenon.

Short-term variability and mass loss in Be stars

“…) with the Solar Mass Ejection Imager (SMEI; Jackson et al 2004) extend from Feb. 2, 2003through Sept. 28, 2011. Contrary to an earlier study in this series using SMEI data (Baade et al 2018a, who also describe SMEI from the stellar-photometry point of view), the data now available identify with which of the three cameras a given measurement was made. This vastly improves the scope of the preparation of the data for time series analysis.…”

“…The rationale for additional processing of the pipeline-reduced data (Popowicz et al 2017) was explained in much detail by Pigulski et al (2018) and Pigulski (2018). For the observations of ν Pup, it was implemented and applied as described in Baade et al (2018a).…”

“…The BRITE and SMEI observations are highly complementary. The former have a much higher fidelity per data point and reveal details of the light curve while the SMEI data extend over a much longer continuous time span and are better suited to study long-term variations and resolve closely spaced features in the power spectrum as has been shown in a number of independent analyses (e.g., Goss et al 2011;Howarth & Stevens 2014;Baade et al 2018a). The noise and the time span of the Hipparcos data are intermediate between those of SMEI and BRITE whereas the number of Hipparcos data points is one to two orders of magnitude smaller.…”

“…Broad consensus exists that the near-critical rotation (Frémat et al 2005;Meilland et al 2012;Rivinius et al 2013) is a necessary condition for the mass loss. Numerous observational studies Huat et al 2009;Baade et al 2017Baade et al , 2018a suggest that much of the star-to-disk mass-transfer process takes place in discrete events, which cover a wide range in cadence and amplitude (Semaan et al 2018;Bernhard et al 2018). Although having the appearance of mass-loss events, they may actually be more accurately described as angular-momentum-loss events.…”

An Observer's View on the Future of Asteroseismology

“…Four large amplitude frequencies were exhibited. Two of them were closely spaced frequencies of spectroscopically confirmed g-modes near 1.5 d −1 , one slightly lower (about 10 %) exophotospheric (Stefl) frequency, and at 0.05 d −1 the difference frequency between the two non-radial g-modes (Baade et al, 2018). The circumstellar (Stefl) frequency did not seem to be affected by the frequency difference, which underwent large amplitude variations.…”